1. Field of the Invention
The present invention generally relates to high speed optical data transmission and more particularly to a data channel with a Vertical Cavity Surface Emitting Laser (VCSEL) diode CMOS driver and a high performance silicon photodetector.
2. Description of the Related Art
It is generally understood that as processor clock rates move further into the multi gigabit per second (Gbps) range, advanced information processing systems may require aggregate communication bandwidths upwards of Terrabit/sec. The typical data interconnect medium, copper wiring, is not expected to be suitable for such bandwidths. Facing a similar problem, the telecommunications and data communications industries have increasingly turned to optics. Thus, it is expected that optics will be used widely in high performance systems, e.g., for optical channels and busses. Optical media will replace electrical in these high performance systems, whether for communicating with a high performance server or in the server itself between frame subassemblies, between individual boards and even between chips on the same board or in the same module. However, most state of the art optical channels require expensive, high performance optical drivers and receivers in compound semiconductors, i.e., group III-V semiconductors such as, GaAs, InP, InGaAs and etc.
Generally, most state of the art high performance systems are based on the complementary insulated gate field effect transistor (FET) silicon technology commonly referred to as CMOS. Typically, much of the performance achieved in high performance communications devices is lost connecting a high performance (fast and sensitive) compound semiconductor element (i.e., a laser diode or a photodetector) to a CMOS driver or receiver. So, in addition to the cost of including a compound semiconductor elements, state of the art laser diode drivers or photoreceivers (i.e., a photodetector driving a receiver) lose much of the performance advantage in the connecting the element to the circuit. Efforts in combining these technologies (i.e., integrating compound semiconductor devices on the same chip with CMOS circuits) have not met with any widespread success and, typically, have proven to be very expensive. So, in spite of their excellent performance advantages, these compound semiconductor photodetectors and photoreceivers have found limited application to date.
State of the art silicon photodetectors also have limited use because of the indirect bandgap character of the silicon, which results in a much lower quantum yield (converting 10-20% of the photons into photocurrent) than direct bandgap compound semiconductors (detecting in excess of 90%). Further, silicon has lower carrier mobilities than compound semiconductors. Consequently, even though typically much more expensive than silicon, primarily, for state of the art high performance (fast and sensitive) photodetectors and laser diodes such as Vertical Cavity Surface Emitting Laser (VCSEL) diodes, compound semiconductors are used for operation at 10 Gbps and higher.
Although not on a practical level, high performance VCSEL diode drive has been achieved with a complex element known in the art as a coaxial radio frequency (RF) bias-T. A typical coaxial RF bias-T is a bulky and expensive element that can be used to drive an individual, discrete VCSEL laser diode. Essentially, the coaxial RF bias-T is reactive, with an input series capacitor driving a parallel output inductor. A bias or offset voltage is applied to one end of the inductor, a driver drives one end of the capacitor and the output at the common connection of the inductor and capacitor drives the laser diode. Thus, by applying an offset bias voltage to the inductor, the laser diode may be biased at or above turn on. An input gating signal passes through the capacitor and is superimposed on a DC bias voltage. Experimentally, such coaxial RF bias-T driven discrete VCSEL diodes have achieved 10 Gbps data rates. Thus, while coaxial RF bias-Ts might provide a laboratory solution for driving a few optical signals; they are not practical for optical busses, where the number of channels and, therefore, the number of VCSEL diodes can be 32, 64, 128 or even as high as 1024. So, because of their expense and bulk, using coaxial bias-Ts outside of the laboratory and particularly for wide channel applications is impractical.
Thus, there exists a need for high speed, low cost optical channels and especially for CMOS VCSEL diode drivers and for high speed, high quantum yield silicon photodetectors and photoreceivers that may be formed on low cost silicon and in particular on CMOS or SOI chips for cheap, simple, high-bandwidth optical interconnections and applications.